The government spokesman told the BBC that troops were finding fewer landmines or booby-trapped cars the deeper they moved into the city.

Mr. Hadia said the car bombs they were finding were smaller than the ones they had used before, "which suggests they were hastily set up."

The troops' biggest fear, he added, was the presence of snipers and the possibility that civilians could be trapped in the battle zone.

Sirte was the hometown of ousted ruler Muammar Gadaffi.

The unity government was formed in Tripoli more than two months ago.

The U.S. said the unity government should be allowed to arm itself against IS. Secretary of State John Kerry has said this would be "the only way to generate the cohesion necessary" to defeat the militants.

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